The need has existed over a long period of time, for a D.C. Infrared System Package with design restraints, such as, small physical size, to sense relatively low temperatures without moving mechanical parts, and with an extremely rapid response. Such a device would need, as a prime concern, good stability with respect to ambient change conditions and the ability to work in a hostile environment which could consist of vibration and heat. Also, this new Non Contact Infrared System must operate with a response time in the range of 50 micro seconds or faster. The Infrared Sensor should be able to sense temperature in the area of 35.degree. C. to about 205.degree. C. Such a sensor would be required to operate in a D.C. mode and with it's optical system produce a spot sensing area as small as about 6 mm at a focal distance of about 300 mm. The wave length would be in the short I.R. wave length of about from 1 to about 3.5 microns.
Those skilled in the art have found many difficulties with available sensors with respect to filling all the requirements; such as, photovoltaic, photo conductive, pyrolectric, and thermovoltaic. Sensors in the art have detectors such as PbSe, PbS and pyrolectric. These usually require a chopping or pulsing frequency to introduce sufficient change in signal response, to achieve low temperature capabilities. However, the speed of response of these devices are much too slow to accomplish the desired end in such an application. The use of Germanium or Silicon detectors would produce a fast response time, but their wave lengths in the infrared spectrum are too short, for example silicon 0.7 to 1.1, germanium 0.7 to 1.8 microns. Both silicon and germanium are not capable of low temperature sensing in the range below 250.degree. C.
The thermopile detector which works on a thermovoltaic effect will produce a good signal at low temperature ranges, but would not produce a small spot size at 300 mm working distance and falls short on the speed requirements. Generally a themopile sensor will produce a speed of response in the milli second range. Both indium antimonide and pyro-electric sensors are consided undersirable do to their chopping requirements, or the need for liquid cooling to keep these detectors stable in a hostile environments. The physical sensor size restraint also eliminates this type of detector.
As a result, the device of the present invention was developed to be extremely fast in response, have a short wavelength range, have the capability of obtaining a small spot size at greater distances, have low temperature sensing capability, and all in a small package design. The device of the present invention utilizes the indium arsenide thermal-electrically cooled, photovoltaic mode infrared sensor. This sensor allows good stability in hostile environments, produces sufficient signal level for the low temperature requirements and allows for a low profile design package, with no moving parts or mechanical chopping. The design package, by the use of thermal-electric cooling utilizing a two stage cooler in a To-8 package, assures small target size and fast response. Such a sensor with thermoelectric-TE cooling is available, from Judson Infrared, Inc. as model #12 T.E. The present invention utilizies a DC amplification of this signal with low off set capabilities. The IC model #LF714 is used as a compensation circuit for Blackbody conditions and to allow the Infrared Sensor to sense target temperatures which are lower than its environment. The device of the present invention also contemplates the use of signal processing and linearization of the Infrared signal to a linearized analog output for display purposes in temperature in degrees Centigrade and a computer input.